Typically, vehicles are equipped with a brake system to provide controlled slowing or stopping of the wheels to halt movement of the vehicle. A common type of brake system is a disc brake assembly associated with the wheels that is actuated by hydraulic or pneumatic pressure generated by an operator of the vehicle depressing a foot pedal. As is known, a disc brake assembly generally includes a rotor secured to the wheel of the vehicle for rotation therewith. The rotor has a pair of opposed friction plates that are selectively engaged by brake shoes supported on opposite sides of the rotor for sliding movement relative thereto.
In operation, the brake pads, which are operatively connected to hydraulically actuated pistons, move between a non-braking position in which they are spaced apart from the opposed friction plates of the rotor and a braking position in which they are moved into frictional engagement with the opposed friction plates of the rotor. In response to actuation by an operator, typically by depressing a brake pedal, the piston urges the brake pads from the non-braking position to the braking position. By this, the brake pads frictionally engage the friction plates of the rotor and slow or stop the rotation of the associated wheel of the vehicle.
To improve braking control and vehicle safety, anti-lock brake systems have been developed. In accordance with these systems, rotation of the wheel is sensed, and the braking response is automatically controlled to avoid skidding situations in which the vehicle wheels lose traction and slide over the pavement rather than engaging the surface at a slower rotational speed.
In a typical anti-lock brake assembly 200 seen in FIGS. 8-11, the rotor 210 is provided with a ring of teeth 212, which are cast with the rotor, commonly referred to as an ABS (Anti-lock Braking System) tone ring. As the rotor 210 rotates, the rotating teeth 212 are read by an anti-lock brake sensor (not shown) that generates a signal for the anti-lock brake control system representative of the rotation of the wheel associated with the rotor 210. The sensor reads the peaks of the teeth and the valleys between adjacent teeth, best seen in FIGS. 10 and 11, and uses an algorithm to determine whether the associated wheel is slipping. If it is determined that the wheel is slipping, braking pressure is released. Obviously, the arrangement and geometry of the teeth influence the signal generated by the sensor. To ensure proper operation of the anti-lock brake system, the teeth must be regularly spaced, sized, and maintained to preserve the profile of the teeth. Many sensors use magnetic pulse generation, which is created as the teeth pass by the sensor. The strength and accuracy of the signal is determined by the magnetic properties of the tone ring and the ring's geometric accuracy. Inadequate magnetic signal strength or incorrect geometric shape may cause signal failure, which can be further influenced by rotating velocity.
Problems have arisen with anti-lock brake systems in terms of poor performance due to irregularities and corrosion of the teeth. In known rotor assemblies in which the teeth are cast with the rotor, the teeth are also subjected to machining and coating treatments that are applied to the rotor. The disc is typically coated with an anticorrosive material, such as Geomet or Magni type coatings that has a friction property and a corrosion resistance property. The coating is intended to lengthen the shelf life of the rotor and impede corrosion. However, since the coating is present when the rotor is put in use and then wears away from the braking surface, the coating must have adequate friction properties so that the rotor functions properly during braking at the outset before the coating is worn off. These dual property constraints limit the possible types of coatings suitable for this application.
Another consideration regarding the coating relates to the teeth. As noted above, the teeth are cast with the rotor, and the coating is applied to the entire piece. However, the teeth involve different design considerations. As the teeth do not function as a friction surface, the friction property of the coating is irrelevant. Further, it is desirable to maintain the anti-corrosive coating on the teeth for the functional life of the assembly. However, coatings suitable for rotor application degrade at high temperatures. This does not pose a problem with respect to the braking surface, but the teeth are exposed to high temperatures during the braking process. Since they are formed integrally with the rotor, which is normally cast iron, they heat to high temperatures, such as 800-900° F., as the rotor heats up due to the heat generated during braking. When the coating breaks down, the teeth can corrode. Corrosion alters the geometry of the teeth and causes inaccurate readings from the anti-lock braking sensor. This significantly shortens the useful life of the brake rotor assembly. When the sensor generates inaccurate readings, the assembly requires repair or replacement.
A problem also exists due to the state of the art casting methods and tolerances, which exist in casting of the teeth. Cast teeth will not be sufficiently accurate for most applications, and the inaccuracy in geometry will cause signal failure at higher velocities. To further machine the teeth for accuracy adds significant additional cost.
Another problem with cast iron tone rings relates to the magnetic properties of cast iron and how the properties change with temperature. Since cast iron has a high carbon content, its magnetism is reduced when heated to high temperatures experienced during braking.
There is a need, therefore, to provide a brake rotor assembly for use with anti-lock brake systems that provides an accurate and durable sensor system. There is also a need for a sensor system that can be retrofit in existing assemblies that no longer provide accurate readings.